Method, mobile exchange, and subscriber station in a mobile radio system for establishing a high-priority call

ABSTRACT

A method, mobile exchange, and subscriber station for establishing a high-priority individual call or group call to at least one subscriber (MS1-MS4) in a mobile radio system participating in a first group call conducted on a given channel. To establish a high-priority call on said channel, a release message provided with an identifier of the subscriber (MS4) participating in the first group call to whom it is desired to establish a high-priority individual call or high-priority group call and/or an identifier of the high-priority individual call or high-priority group call to be established is sent (405) on said channel to all subscribers (MS1-MS4) engaged in the first group call; the subscribers (MS1-MS4) engaged in the first group call receive said release message; said subscriber (MS4) to whom it is desired to establish a high-priority individual call or high-priority group call detects (503, 507) on the basis of said identifier included in the release message received that the release message is intended for said subscriber (MS4); said subscriber (MS4) terminates (506) his engagement in the group call; said subscriber (MS4) joins said high-priority call on the basis of the high-priority call identifier received.

FIELD OF THE INVENTION

The invention relates to a method for establishing a high-priorityindividual call or group call to at least one subscriber in a mobileradio system participating in a first group call conducted on a givenchannel.

The method of the invention is specifically designed for use in what areknown as trunked networks, which are typically company networks orgovernment networks in which all channels are shared by one or severalcompanies or governmental agencies. In such networks the subscribershave individual subscriber numbers and also group numbers indicating thegroup call group or subscriber group the subscriber belongs to, thusenabling calls intended for members of a given group to be switched tosubscribers of said group.

The invention is suitable to be used in mobile radio systems withdigital as well as analog radio paths. Analog mobile radio systems havebeen described for example in MPT 1327, January 1988, revised andreprinted November 1991, A Signalling Standard for Trunked Private LandMobile Radio Systems, and MPT 1343, January 1988, revised and reprintedSeptember 1991, Performance Specification, United Kingdom Department ofTrade and Industry, Radiocommunications Agency.

BACKGROUND OF THE INVENTION

In public mobile radio networks, such as the NMT (Nordic MobileTelephone) system, all calls have the same priority, i.e. none of theusers have rights taking precedence over the rights of other users.

In private radio networks, some of which implement the trunked method,the situation is different. In such PMR (Private Mobile Radio) networks,it is possible to control the operation of the network in such a waythat users carrying out specific tasks, e.g. emergency servicing orother urgent work, have a higher priority, i.e. their high-prioritycalls are the first to be switched in a heavily loaded trafficsituation. It is even possible to define so high a priority that anexisting call is cleared down before the new call if the new call cannototherwise be switched. In most cases, the reason for switching beingprevented is the fact that all radio channels are occupied, or the factthat the called subscriber is already engaged in a call. Other resourcesmay also in fact be occupied and prevent switching of the call.

Where two-party calls are in progress in a mobile radio system, thesystem--for example its mobile exchange or a database in the mobileexchange--is aware of which radio units participate in the call. On thisbasis, when a high-priority call is to be established to a subscriberengaged in a call, the mobile exchange can terminate the ongoing call todisengage the subscriber to whom a new call is to be established. In agroup call the situation is more difficult, as the mobile exchange inthe mobile radio system is not necessarily aware of which group callsgiven subscribers belong to, and thus the mobile exchange is not awareof which radio channel said subscribers are listening to.

A group call is a call in which the system commands several, at leastthree, subscribers to the same call with a single command. The groupsare formed of members of a company or other user organization, and oftenthe groups are programmed into the mobile radio and the mobile exchangeis not necessarily aware of which mobile radios belong to each group.Typically one mobile radio may belong to several groups. Part of thegroup numbers may even be such that they may be changed very often, ifnecessary every morning, and an indication of these changes is notnecessarily sent to the mobile exchange. For example the MPT 1327 andMPT 1343 standards do not provide for means for communicating the groupnumbers from the mobile radios, i.e. subscriber stations, to the mobileexchange. This makes it especially difficult to include a subscriber whois already engaged in a previously initiated group call in ahigh-priority call. This situation is very common in mobile radionetworks, as very often specifically those subscribers who are wanted inhigh-priority calls already participate in prior group calls on accountof the fact that said subscribers have a vital function in theirorganizations.

The above problem has been resolved in prior art solutions by clearingdown all group calls within the area of the mobile radio system. Byclearing down the group calls, the subscriber who is desired to beincluded in the high-priority call can in fact be found, but at the sametime the initial, i.e. first, group call is terminated and it must bere-established without the party that has been included in thehigh-priority call. This procedure causes considerable disadvantage toother subscribers in the mobile radio system, as these must undulyinterrupt their group call. When interrupted group calls arere-established, the mobile radio system is loaded by unnecessaryconnection establishment procedures. Furthermore, if the mobile radiosystem has a very high traffic level, the initial group call group mayforfeit the radio channel it used, as another group call or another callmay seize the channel while the first group call is interrupted and oneof its subscribers is being searched for a high-priority individual callor group call.

A further drawback of the prior art solution is that if it is notdesired to clear down the existing group call, the subscriber wanted forthe high-priority call cannot be included in the new call to beestablished at all. Thus the subscriber has poor availability.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a method forestablishing a high-priority individual call or group call to asubscriber participating in a first group call conducted on a givenchannel which avoids the problems involved in the solutions according tothe prior art. The object is to be able to release a subscriber engagedin a group call to participate in a high-priority call without theproblems entailed by the solution according to the prior art. It isanother object of the present invention to enable inclusion of a givensubscriber in a given high-priority call. It is a further object torealize a method for establishing a high-priority individual call orgroup call, according to which method as one or more subscribers engagedin a group call leave the group call, the subscribers remaining with theinitial group call are given the opportunity to become aware that one ormore identified subscribers are leaving or have left said call.

This novel method for establishing a high-priority individual call orgroup call to at least one subscriber in a mobile radio systemparticipating in a first group call conducted on a given channel isachieved with the method of the invention, which is characterized inthat a release message provided with an identifier of the subscriberparticipating in the first group call to whom it is desired to establisha high-priority individual call or high-priority group call and/or anidentifier of the high-priority individual call or high-priority groupcall to be established is sent on said channel to all subscribersengaged in the first group call; the subscribers engaged in the firstgroup call receive said release message; said subscriber to whom it isdesired to establish a high-priority individual call or high-prioritygroup call detects on the basis of said identifier included in therelease message that the release message is intended for saidsubscriber; said subscriber terminates his engagement in the group call;said subscriber joins said high-priority call on the basis of thehigh-priority call identifier received.

The present invention further relates to a mobile exchange comprising aswitching matrix, an interface unit, a controller, and memory means forstoring subscriber identifiers and group call identifiers. The mobileexchange of the invention is characterized in that it comprises meansfor sending a release message provided with an identifier of thesubscriber participating in the first group call to whom it is desiredto establish a high-priority individual call or high-priority group calland/or an identifier of the high-priority individual call orhigh-priority group call to be established, to all subscribers engagedin the first group call.

The invention further relates to a subscriber station in a mobile radiosystem, comprising a memory for storing identifiers of group callsallowed for the subscriber station, a transceiver, a handset, a controlunit for controlling the operation of the subscriber station in responseto commands from the handset and the system controller. The subscriberstation in a mobile radio system in accordance with the invention ischaracterized in that the subscriber station further comprises means forreceiving a release message and for comparing the identifier of thehigh-priority individual call or high-priority group call to beestablished, included in the release message, with the identifier of thesubscriber station and/or with a group call identifier stored in thememory of the subscriber station as an identifier of an allowed groupcall, means responsive to such comparison for releasing the subscriberstation from the first group call and for transferring the subscriberstation to a high-priority individual call or high-priority group callto be established.

The invention is based on the idea that a subscribed engaged in a groupcall can be released for a high-priority call by sending a releasemessage on the channels for all group calls either to said subscriber ifan individual call is to be established, or to a group if a group callis to be established. The mobile radio must be capable of processing therelease message that has arrived in such a way that the mobile leavesthe group call. The other subscribers can remain with the group callunless the channel must be seized for the call to be established. Inthat case, the given subscriber or subscribers are released from theongoing group calls. This is naturally only the case when saidsubscribers are within the service area of the mobile radio system. Ifthey are not within its service area, receipt of the release message hasno significance, since outside the service area the subscribers wouldnot be able to hear the call to enter the new call either. If thetraffic channel that is used is to be cleared for a new call, therelease message is sent using the same call identifier by means of whichthe group call was established, since all subscribers must leave saidgroup call.

It is an advantage of such a method, mobile exchange, and subscriberstation in a mobile radio system for establishing a high-priorityindividual call or high-priority group call that the method, mobileexchange and subscriber station of the invention improve theaccessibility of a subscriber engaged in a group call when such asubscriber is wanted in a high-priority call and when it is notnecessarily desired to clear down the group call in which saidsubscriber is engaged.

A further advantage of the invention is that it is not necessary toclear down the group call entirely when it is desired to establish ahigh-priority call to one subscriber participating in the group call, asin the prior art solution. Thus the other subscribers participating insaid group call are not unduly excluded from the group call but cancontinue the group call without disruption, even though one or moresubscribers engaged in the group call transfer to a high-priority groupcall.

This entails a further advantage of the invention, as it avoidsestablishment of a new group call to those subscribers participating inthe group call who have not been called to enter a new high-prioritycall.

The fact that the earlier group call need not be cleared down entailsthe advantage that connection establishment messages necessary forestablishing a new group call, which would load the mobile radio system,need not be sent in the system.

Still another advantage of the invention is that since the earlier groupcall is not cleared down, the risk of the channel for the earlier groupcall being allocated for the needs of another call while the group callis interrupted is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained in greater detail withreference to the accompanying drawings, in which

FIG. 1 is a schematic view of subscriber stations in a group call,

FIG. 2 is a block diagram of a mobile exchange of the invention for amobile radio system,

FIG. 3 is a block diagram of a subscriber station of the invention for amobile radio system,

FIG. 4 is a flow chart of the operation of the mobile exchange of theinvention,

FIG. 5 is a flow chart of the operation of the subscriber station of theinvention,

FIG. 6 is a block diagram of an embodiment of the subscriber station ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of subscriber stations in a group call. Theschematic view shows a mobile exchange MX to which base stations BS1-BS4are connected by means of telecommunication connections. The figure alsoshows subscribers MS1-MS6 located within the service area of the mobileexchange, accessing via radio paths to base stations BS1-BS4. Thesubscriber stations, mobile radios MS1-MS4 participate in a first(former) group call by listening to the channel for said group call.Presently the mobile exchange receives information on the fact thatmobile station MS4 is wanted in another call having high priority, whichmay be a normal two-party call MS4-MS5 or a group call between severalsubscribers MS4, MS5, MS6. In that situation, the mobile exchange MXsends a release message to subscriber stations MS1-MS4 on the channelsfor all group calls. The release message may be, in trunked systems forexample, a normal release message MAINT (clear) to be sent on a trafficchannel. This message is detailed in the MPT 1327 and MPT 1343specifications. The invention, however, comprises the novel featuredistinct from the MPT 1327 and MPT 1343 specifications that during agroup call, release messages are sent that are not related to thatparticular group call but to new calls to be established. The callcontrol processor at the mobile exchange makes the decision regardingthe sending of a release message, and the call control processor is alsoin charge of the new call to be established. The subscriber station MS4,which is wanted in a high-priority call instead of the group call,detects from the release message that he is wanted in the newhigh-priority call. Thereupon said subscriber MS4 terminates hisparticipation in the "old" group call and starts listening to thechannel for the new group call or initiates a new call with anothersubscriber station MS5.

It is to be noted that the mobile radio system may have simultaneouslyseveral group calls established, and release messages commanding thegiven subscribers to terminate their participation in said group callsand to enter new calls of high or higher priority, which may be newgroup calls or new two-party calls, can be sent to all subscriberstations participating in the previous calls on the channels for suchgroup calls.

The release message must be sent to subscribers in all currentlyestablished group calls, as it is not known which group call saidsubscriber participates in or whether the given (sought) subscriber isengaged in any ongoing group call. Sending of the release messageinduces slight deterioration in voice quality, which can be compensatedfor almost entirely by present-day techniques.

FIG. 2 is a block diagram of a mobile exchange 300 of the invention fora mobile radio system. It is the task of interface unit 304 to offerinterfaces 305 for base stations BS1, BS2, BS3, BS4 for voice andsignalling traffic. There may also be other interfaces, such asinterfaces to a public switched telephone network, PABXs and otherexchanges 307. A switching matrix 301 and a controller 302 are connectedto the interface unit 304, for example by way of a data bus.

It is the task of the switching matrix 301 to switch speech pathsbetween the various subscriber units call-specifically. In a two-partycall, the speech paths of two subscriber units are interconnected. In agroup call, all subscriber stations MS1, MS2, MS3, MS4, MS5, MS6belonging to the call are interconnected, for example in aconference-type connection.

The controller 302 attends e.g. to the set-up and clearing down ofcalls. For this purpose, the controller is connected to the interfaceunit 304 by way of which it can send and receive signalling messages.The controller 302 is also connected to the switching matrix 301, whichit controls call-specifically. In the mobile exchange 300 of theinvention, means 306 are incorporated in the controller 302 for sendinga release message to all subscribers MS1, MS2, MS3, MS4 participating inthe first group call. Said means 306 may send the release message toparticipants in other group calls as well. The release message may beprovided with an identifier identifying a subscriber M4 participating inthe first group call, to whom it is desired to establish a high-priorityindividual call or high-priority group call. The release message may befurther provided with an identifier identifying the high-priorityindividual call or high-priority group call to be established. Thesubscriber station compares the identifier included in the releasemessage with its own identifier, and if the identifier of the releasemessage and that of the subscriber station are the same, the subscriberstation terminates its participation in the group call and returns tothe control channel, whereafter the subscriber station Joins the givenhigh-priority call. The subscriber station performs a similarcomparison, and if the release message contains any of the group numbersof the subscriber, the subscriber station terminates its participationin the earlier group call and Joins the high-priority group call.

The controller 302 also comprises memory means 303. This memory means303 may be either incorporated in the controller 302, incorporated inthe entire mobile exchange, or implemented as a separate memory means.All information necessary for the operation of the mobile radio systemis stored in the memory means 303. This information may includeinformation related to the subscriber stations MS1, MS2, MS3, MS4, MS5,MS6, for example information on subscribers entitled to communicate inthe system, information on the functions allowed for each subscriberstation MS1, MS2, MS3, MS4, MS5, MS6, information on the location of thesubscriber stations, for example with an accuracy of one base station orlocation area, and information on the ongoing calls.

FIG. 3 illustrates a typical subscriber station, i.e. the configurationof a mobile radio used by a subscriber. It is the task of transceiver(TX/RX) 501 to tune to the radio channel employed in each case. Thetransceiver incorporates an antenna 502 connected to a radio path RP.Normally radio frequencies in the range 60-1000 MHz (VHF and UHF ranges)are employed, although other frequencies may be used as well. Analogmodulation may be used in the radio path RP, the mode of modulationnormally being phase modulation. Also other modes of modulation may beused. A voice-frequency subcarrier (FFSK), for example, may be used forsignal transmission. The radio path may also be conveyed digitally.

The handset 505 comprises electroacoustic transducer means, typically anearphone 506 and a microphone 509, and possibly push buttons in the userinterface 507 for the initiation and termination of the call as well asselection. Since in a trunked system speech in the radio path RP ispreferably alternate, the handset normally also has a press-to-talk(PTT) switch which must be pressed during the speaking turn.

It is the task of the control unit 503 to control the function of thesubscriber station. The control unit 503 has access to the handset 505wherefrom it obtains the impulses for the initiation and termination ofa call, for instance. The control unit 503 may also give to the user anaudio signal or a visual signal related to the mobile radio and/or themobile radio system through the handset 505 or user interface 507.

The control unit 503 is connected to the transceiver TX/RX 501. Thechannel used by the transceiver is determined by the control unit 503,in other words, the transceiver 501 tunes to the channel (radiofrequency) determined by the control unit 503. The transceiver 501 isalso switched on under the control of the control unit 503. The controlunit 503 receives and sends signalling messages through the transceiver501. The control unit may also receive an indication of the signalstrength of the received signal.

The control unit 503 also incorporates a memory 504, storing permanentdata and volatile data. Such permanent data includes the individualnumber of a subscriber station, numbers of the group call groups inwhich the subscriber station is allowed to participate, i.e. theidentifiers of the group calls allowed for the subscriber station, andradio channels belonging to the system. Said volatile data includes themomentary status of the subscriber station, e.g. idle state, call setupstate, in-call state, etc., and for example information on which channelthe subscriber station is tuned to.

The subscriber station 500 according to the invention further comprisesmeans 508 for receiving a release message sent by the mobile exchange300 and for comparing the identifier included in the release messagewith the identifier of the subscriber station and/or a group callidentifier recorded as an identifier for an allowed group call in thememory 504 of the subscriber station. The release message sent by themobile exchange includes the identifier of the high-priority individualcall or high-priority group call, which the subscriber station compareswith its own identifier stored in the memory or with the identifier ofan allowed group call. After the comparison, means 509 responsive tosaid comparison release the subscriber station from the first group calland connect the subscriber station with the high-priority individualcall or group call to be established, if the result of the comparison ispositive, i.e. the identifier included in the release message is thesame as the identifier stored in the memory of the subscriber station.

FIG. 4 is a flow chart of the operation of the mobile exchange of theinvention. The figure illustrates how a subscriber engaged in a groupcall is commanded to enter an emergency call, i.e. a highly priorizedcall. The description of FIG. 4 herein is from the point of view of themobile exchange 300 or system controller of the mobile radio system.Initially, the mobile exchange or its system controller awaits 401 arequest for a call to be switched. The mobile exchange checks 402whether the call to be switched is an emergency call or otherhigh-priority call. If the inbound call is of a high priority, themobile exchange sends 405 a release message to subscribers wanted insaid call on the traffic channels of all ongoing group calls.Thereafter, and also if the call to be connected does not have 404 ahigh priority, i.e. is not an emergency call, the mobile exchange sendsa command 406 to the given subscriber for the new call on the controlchannel, whereafter the mobile exchange returns to await switching ofthe new call to step 401.

FIG. 5 is a flow chart of the operation of the subscriber station 500 ofthe invention. The figure shows how a subscriber engaged in a group callis commanded to enter an emergency call, i.e. a highly priorized call.The description here is from the point of view of the subscriberstation, i.e. the subscriber unit (mobile radio). The subscriber stationawaits 701 to be commanded to the call. When the subscriber station hasbeen commanded to the call, typically a group call, the subscriberstation awaits 702 a release message sent by the mobile exchange. Uponreceipt of a release message, the subscriber station checks 703 whetherthe release message is for its subscriber number or individual number.If the release message includes 704 the subscriber number of saidsubscriber station, the subscriber station leaves the group call inwhich it has participated and returns to the control channel. If, on theother hand, the release message does not include 705 the subscribernumber of said subscriber station, the subscriber station checks whetherthe release message is intended for any of the group numbers of thegiven subscriber, in other words, the subscriber station performs acomparison as to whether the group call identifier included in therelease message is the same as the identifier of any allowed group callstored in the memory of said subscriber station. If this is the case,the operation proceeds with step 706, as set out above. If, on the otherhand, the release message does not include 708 identifiers of the groupnumbers of the subscriber station, the subscriber station moves to awaita new release message 702.

FIG. 6 is a block diagram of an embodiment of the subscriber station ofthe invention. The signalling controller 604 of the subscriber stationinterprets the incoming messages from the radio path, and respectivelygenerates the outgoing messages and the necessary acknowledgementmessages. The signalling controller 604 further receives incomingrelease messages and checks whether they are intended for a givensubscriber, either to his individual number or to any of his groupnumbers. A network supervisor 603 maintains contact with the mobileradio system. A FFSK modem 602 generates from the messages arriving fromthe signalling controller an audio signal for the radio 601, said signalmodulating the RF-frequency (80 MHz, 160 MHz or 450 MHz) carrier of theradio. User interface 605 is the interface between a terminal and theuser.

The invention further comprises a method for speeding call establishmentfor a call to be set up. In the method, a release message by which asubscriber is released from the previous call may include information inresponse to which the subscriber immediately moves to a new channel. Inthat case, the release message is a message causing the subscriberimmediately to move to listen to a new radio channel.

In implementing the inventive method for establishing a priority call asoutlined above, a situation may arise where one or several subscribersleave a lower-priority call for a higher-priority call. In such asituation, the other subscribers belonging to the initial group call donot become aware of the fact that said one or more subscribers have leftthe call, but assume that they are still engaged in the call and attemptto communicate with them.

A way to solve the above problem is to clear down all calls in whichparticipants for the new call to be established are engaged. This stepis naturally too extreme and also unnecessary, as it brings disadvantageof the kind set out in connection with the description of precedingdrawbacks of invention.

Another way to solve the above problem is to use a call waiting functionimplemented to the participants for the new call to be establishedbefore they are released from the initial call. In that case, when thesubscribers to be transferred to the new call are signalled, by using acall waiting function, that they are about to be transferred to a newcall, they can inform the subscribers remaining with the initial callorally on the group call channel that they will transfer to communicateon a new channel. The above solution is not suitable for emergency callsor other high-priority calls, since it requires a slow user-originatedoperation from several subscribers who possibly attempt to perform italmost simultaneously, and thus the initial channel is unnecessarilyloaded and passing of information is further slowed down.

Therefore, in operating according to an additional embodiment of theinvention, the release message to be sent in accordance with theinvention includes information by means of which the subscriber orsubscribers to be released from the initial call are informed that theywill move to another call. Furthermore, also the subscribers remainingwith the initial call can be informed, by means of said release message,about the subscribers that transfer to the new call. Thus in theadditional embodiment of the invention, the release message includesinformation in response to which the subscriber MS4 is informed that hewill be transferred to a given high-priority individual call orhigh-priority group call. The release message may further includeinformation in response to which the subscribers remaining with theinitial call or group call are informed that the subscriber who iswanted in a high-priority individual call or a high-priority group callwill be transferred to said high-priority individual call orhigh-priority group call.

The drawings and the description referring to them are only intended toillustrate the idea of the invention. The method, mobile exchange, andsubscriber station in a mobile radio system for establishing ahigh-priority individual call or high-priority group call in accordancewith the invention may vary in their details within the scope of theclaims. Even though the invention has been set forth in the aboveprimarily in the context of trunked systems, it may be implemented inother kinds of radio telephone and mobile telephone systems as well.

I claim:
 1. A method for establishing a high-priority individual call orgroup call to at least one subscriber in a mobile radio systemparticipating in a first group call conducted on a given channel, themethod comprising the steps of:sending a release message provided withan identifier of the subscriber participating in the first group call towhom it is desired to establish a high-priority individual call orhigh-priority group call and/or an identifier of the high-priorityindividual call or high-priority group call to be established on saidchannel to all subscribers engaged in the first group call, receiving bythe subscribers engaged in the first group call of said release message,detecting by said subscriber to whom it is desired to establish ahigh-priority individual call or high-priority group call, on the basisof said identifier included in the release message, that the releasemessage is intended for said subscriber, terminating by said subscriberof said subscriber's engagement in the group call, and joining by saidsubscriber of said high-priority call on the basis of the high-prioritycall identifier received.
 2. A method as claimed in claim 1, wherein therelease message is sent to all subscribers participating in ongoinggroup calls in the mobile radio system.
 3. A method as claimed in claim1, wherein the release message includes information in response to whichthe subscriber that has received the release message immediately movesto listening to the channel for the call to be established.
 4. A methodas claimed in claim 2, wherein the release message includes informationin response to which the subscriber that has received the releasemessage immediately moves to listening to the channel for the call to beestablished.
 5. A method as claimed in claim 1, wherein the releasemessage includes information in response to which the subscriber isinformed that the subscriber will be transferred to said high-priorityindividual call or said high-priority group call.
 6. A method as claimedin claim 2, wherein the release message includes information in responseto which the subscriber is informed that the subscriber will betransferred to said high-priority individual call or said high-prioritygroup call.
 7. A method as claimed in claim 3, wherein the releasemessage includes information in response to which the subscriber isinformed that the subscriber will be transferred to said high-priorityindividual call or said high-priority group call.
 8. A method as claimedin claim 4, wherein the release message includes information in responseto which the subscriber is informed that the subscriber will betransferred to said high-priority individual call or said high-prioritygroup call.
 9. A method as claimed in claim 1, wherein the releasemessage includes information in response to which the subscribersremaining with the initial group call are informed that the subscriberto whom it is desired to establish a high-priority individual call orhigh-priority group call will be transferred to said high-priorityindividual call or high-priority group call.
 10. A method as claimed inclaim 2, wherein the release message includes information in response towhich the subscribers remaining with the initial group call are informedthat the subscriber to whom it is desired to establish a high-priorityindividual call or high-priority group call will be transferred to saidhigh-priority individual call or high-priority group call.
 11. A methodas claimed in claim 3, wherein the release message includes informationin response to which the subscribers remaining with the initial groupcall are informed that the subscriber to whom it is desired to establisha high-priority individual call or high-priority group call will betransferred to said high-priority individual call or high-priority groupcall.
 12. A mobile radio system for establishing a high-priorityindividual call or group call to at least one subscriber in a mobileradio system participating in a first group call conducted on a givenchannel, the mobile radio system comprising:a mobile exchangecomprising:a switching matrix; an interface unit; a controller; a memoryfor storing subscriber identifiers and group call identifiers; andsending means for sending a release message provided with an identifierof the subscriber participating in the first group call to whom it isdesired to establish a high-priority individual call or high-prioritygroup call and/or an identifier of a high-priority individual call orhigh-priority group call to be established, to all subscribers engagedin the first group call; and subscriber equipment of subscribers engagedin the first group call being arranged to receive said release message,wherein:said subscriber equipment of said subscriber to whom it isdesired to establish a high-priority individual call or high-prioritygroup call being arranged to detect on the basis of said identifierincluded in the release message that the release message is intended forsaid subscriber; said subscriber equipment of said subscriber beingarranged in response to said detection to terminate the engagement ofsaid subscriber in the group call; and said subscriber equipment of saidsubscriber being arranged to join said high-priority call on the basisof the high-priority call identifier received.
 13. A subscriber stationfor a mobile radio system, comprising:a memory for storing identifiersof group calls allowed for the subscriber station; a transceiver; ahandset; a control unit for controlling the operation of the subscriberstation in response to commands from the handset and a system controllerof the mobile radio system; receiving means for receiving a releasemessage containing an identifier of high-priority individual call orhigh-priority group call to be established, for comparing the identifierincluded in the release message, with the identifier of the subscriberstation and/or with a group call identifier stored in the memory of thesubscriber station as an identifier of an allowed group call, and fordetecting on the basis of said identifier included in the releasemessage that the release message is intended for said subscriber; andterminating means responsive to such comparison for terminating theengagement of the subscriber in the first group call and for releasingthe subscriber station from the first group call and for transferringthe subscriber station to a high-priority individual call orhigh-priority group call to be established.